Image Formation and CT Numbers

The various shades of gray that make up a CT image are determined by the density of a structure and the amount of x-ray energy that passes through it. This phenomenon is referred to as the attenuation of the x-ray. The degree of beam attenuation on a CT image is quantified and expressed in Hounsfield units (HU), which are also referred to as CT numbers. Attenuation values span a range of 4000 CT numbers, from air at -1000 HU, to cortical bone at +3000 HU, and water is assigned the density of approximately 0 HU.

In conventional CT, each cross-sectional slice through a patient's body has a thickness, referred to as its z axis. The data are then further divided into tiny cubes of equal volume called voxels (volume elements). Each voxel is assigned a CT number that is determined by the degree to which the material in that voxel absorbed the x-ray beam. The two-dimensional CT image is formed by displaying the front face of each voxel, termed a pixel (picture element), in a composite matrix. The most common matrix size of CT scanners is 512 rows of pixels by 512 columns, or a total of 262,144 pixels1 (Fig.296:1).

FIG. 296-1. The gray disc represents a cross-sectional slice corresponding to the patient. To create an image, the patient's data are segmented. A pixel is a two-dimensional square. A voxel incorporates the thickness of the slice and is a three-dimensional cube.

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